1. Field of the Invention
This invention relates to dynamic magnetic information storage and retrieval and, ore particularly, to heads which are magnetoresistive, and to sensors of magnetic fields in general.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 3,864,751 of Beaulieu et al for an "Induced Bias Magnetoresistive Read Transducer" shows a magnetoresistive sensor 10 deposited on an insulating layer 12 on a magnetic shield 20. In FIG. 5 of that patent a shunt bias layer 26 of titanium can be provided. The magnetoresistive sensor 10 and the layer 26, if present, are then covered by SiO.sub.2 insulating layers 14 or 27 on which is deposited a bias film 16 of Permalloy. Thus, the magnetoresistive sensor or the layer 26 is separated from the bias layer by SiO.sub.2 which we have discovered can have spurious electrical breakdowns causing variations in resistance which produces spurious signals and can lead to degraded magnetic and electrical characteristics. Previously, it had been thought, as shown by Beaulieu et al and commonly assigned Voegeli U.S. Pat. No. 3,860,965, that the normal provision of insulation was necessary probably in order to prevent degradation of the signal in Beaulieu et al and in order to function in the case of the Voegeli patent and in a publication by O'Day entitled, "Balanced Magnetic Head," IBM Technical Disclosure Bulletin, Vol. 15, No. 9, Feb. 1973, p. 2680.
In addition, Beaulieu et al provides a structure which makes it very difficult to provide contact with the lower magnetoresistive sensor because the conductor requires a discontinuity in the structure, causing Barkhausen noise and potential short circuits.
The fact has been discovered by experimentation that if insulation is used, it must be on the order of 1000A thick or thicker to be reliable whereas magnetic efficiency considerations require an insulation on the order of 200A thick, which leads to the unreliability and degradation characteristics referred to above.
The O'Day U.S. Pat. No. 3,814,863 and Brock et al U.S. Pat. No. 3,813,692 are commonly assigned and both show an MR layer deposited on a substrate covered with a layer of titanium which does not rely on magnetic bias but which uses current flowing through the titanium film to provide the bias instead. This form of bias is less effective because it requires much larger bias currents, thus producing excessive heat, and is critically dependent upon spacing in the gaps.